Rick Santorum swept three nominating contests held Tuesday, upsetting frontrunner Mitt Romney and injecting new energy into the former Pennsylvania senator's campaign.

Santorum scored broad victories in the Minnesota caucus and a primary in Missouri, according to NBC News projections. But Santorum's most significant upset came in Colorado, where the state GOP declared him the apparent victor in caucuses there.

Romney made his hardest push of the three states in Colorado, having campaigned there and spent money on advertising. Santorum's upset raises fresh doubts about the breadth of Romney's appeal to Republicans, and abates some of the momentum Romney had built from consecutive victories in the Florida primary and Nevada caucus.

Still, the former Pennsylvania senator drew contrasts with Romney throughout his remarks, saying Romney "has the same positions as Barack Obama" on a number of issues close to conservatives. Santorum also made a disapproving nod toward Romney's gaffe last week in which the former Massachusetts governor said his campaign was "not concerned about the very poor."

"I care about the very rich and the very poor," Santorum told supporters. "I care about 100 percent of America."

The Romney campaign had begun to downplay expectations for its finish in Missouri and Minnesota, though the Colorado finish seemed more genuinely surprising to Romney. Throughout the campaign, Romney, the tentative frontrunner, has been dogged by questions about his ability to close the deal with Republicans — questions that will be furthered by Tuesday night's returns.

Romney conceded in remarks just before midnight in the East Coast that "this was a good night for Rick Santorum."

Rick Santorum swept Tuesday's Republican presidential contests in Colorado, Minnesota and Missouri, sending a signal to Romney that voters are still skeptical of his conservative credentials. NBC's Peter Alexander reports.

Santorum had campaigned last week in Missouri, Minnesota, and Colorado — while Romney, Newt Gingrich and Texas Rep. Ron Paul campaigned in Nevada. Since Saturday's Nevada caucuses, Romney has basically campaigned only in Colorado.

While NBC News will not project allotments of delegates based off the results, Santorum's sweep provides a new springboard for his campaign heading into a crucial stretch for the campaign.

The former Massachusetts governor had been seen as a marginal favorite in the contests simply because of his organizational strength. Romney had also won the 2008 Minnesota caucus. This cycle, he fell to third, behind Santorum and Paul.

The winner of Jan. 3's Iowa caucuses by a razor-thin margin, Santorum had focused on winning over the same brew of social conservatives and Republicans not ready to settle for Romney during his campaign efforts.

"Tonight's victory should put to bed the idea that the Republican nomination for Mitt Romney is inevitable," said Stuart Roy, an adviser to the pro-Santorum super PAC, the Red, White and Blue Fund.

The victories, however, are somewhat informal. Missouri will host a separate caucus next month to allocate its delegates, and the Minnesota and Colorado caucus results are non-binding.

Romney had been looking to keep alive an unbeaten streak, which started last Tuesday in Florida and continued through Saturday night's Nevada caucus. But his campaign started to play down expectations for his performance in these contests after signs of momentum for Santorum had begun to emerge.

"Of course, there is no way for any nominee to win first place in every single contest … and we expect our opponents to notch a few wins too," Romney political director Rich Beeson wrote in a memo to reporters. "It is difficult to see what Governor Romney’s opponents can do to change the dynamics of the race in February."

Romney said earlier in the evening that he was "pretty confident" he'd finish first or second in Colorado, which also hosted caucuses Tuesday evening, before adding that he expects to become the GOP nominee when the primary concludes.

Romney's campaign had additionally waged an offensive against Santorum late in the weekend, looking to stymie his climb much as they had done with Gingrich in Florida.

Santorum's late surge undercuts Romney's claim to being the GOP campaign's sole frontrunner. He'd sought to cruise through the lighter schedule in February. By contrast, Santorum had the most to gain from proving he can upset the former Massachusetts governor. Alternatively, had Santorum been unable to beat Romney despite his intense focus on these contests, it would have raised questions about his viability going forward.

"Gov. Romney is uniquely unqualified to take on the most important issue in this election," Santorum said yesterday in Rochester, referring to the health care reforms Romney had supported in Massachusetts. "Gov. Romney is dead wrong on the most important issue of the day and he should not be our nominee."

The Romney campaign saw its best chances tonight in Colorado, where they have spent money on advertising, and where Romney has done most of his campaigning since winning in Nevada.

But having been injected with a new twist, the Republican primary campaign is set to move forward with no ending on the horizon, meaning Republicans' focus will remain more on each other than the general election match-up against President Obama.

Paul has signaled that his campaign will continue to focus on select caucuses, reflecting their sense that Paul is best positioned to pick up delegates in those kinds of contests. The libertarian-minded congressman stressed the delegate battle in remarks late Tuesday evening.

GOP presidential candidate Ron Paul thanks supporters for their effort following a strong showing in that state.

Arizona and Michigan's primaries are the next test for the candidates; Romney was raised in Michigan, where his father also served as governor. He's seen as the early favorite in the Wolverine State.

The biggest point of emphasis, though, will come on March 6 -- Super Tuesday. A number of large states host primaries and caucuses that day, and the candidates are already turning their attention to those states.

Gingrich, for instance, spent Tuesday campaigning in Ohio, the key swing state which hosts its primary on Super Tuesday.

Santorum, meanwhile, is expected to continue on Wednesday to Texas, whose primary date is in flux due to litigation over the state's congressional redistricting map.